Question
List the major digestive enzymes, their site of action, substrate, product, and optimal pH. Why does pepsin work in the stomach but not in the intestine?
(NEET + CBSE Board — table + reasoning)
Solution — Step by Step
| Enzyme | Source | Site of Action | Substrate | Product | Optimal pH |
|---|---|---|---|---|---|
| Salivary amylase | Salivary glands | Mouth | Starch | Maltose | 6.8 (slightly acidic) |
| Pepsin | Gastric glands (chief cells) | Stomach | Proteins | Peptones, peptides | 1.5-2.0 (highly acidic) |
| Trypsin | Pancreas | Duodenum | Proteins, peptones | Peptides | 7.5-8.5 (alkaline) |
| Chymotrypsin | Pancreas | Duodenum | Proteins | Peptides | 7.5-8.5 |
| Lipase (pancreatic) | Pancreas | Duodenum | Fats (emulsified) | Fatty acids + glycerol | 7.5-8.5 |
| Pancreatic amylase | Pancreas | Duodenum | Starch | Maltose | 7.5-8.5 |
| Maltase | Intestinal glands | Small intestine | Maltose | Glucose | 7.0 |
| Sucrase | Intestinal glands | Small intestine | Sucrose | Glucose + fructose | 7.0 |
| Lactase | Intestinal glands | Small intestine | Lactose | Glucose + galactose | 7.0 |
| Nuclease | Pancreas/intestine | Small intestine | Nucleic acids | Nucleotides | 7.5 |
Pepsin is secreted as an inactive precursor called pepsinogen by chief cells. The highly acidic HCl (pH ~2) in the stomach converts pepsinogen to active pepsin by removing a small peptide fragment.
Pepsin’s active site is shaped to function optimally at pH 1.5-2.0. When food moves to the duodenum, the pH jumps to 7.5-8.5 (due to bicarbonate from the pancreas). At this pH, pepsin is denatured — its 3D structure unfolds and it loses catalytic activity. Trypsin takes over protein digestion in the intestine.
Many digestive enzymes are secreted as inactive proenzymes to prevent self-digestion:
- Pepsinogen → Pepsin (by HCl)
- Trypsinogen → Trypsin (by enterokinase)
- Chymotrypsinogen → Chymotrypsin (by trypsin)
- Procarboxypeptidase → Carboxypeptidase (by trypsin)
This cascade ensures enzymes activate only in the right location.
graph TD
A["Food Enters"] --> B["Mouth: Salivary Amylase"]
B --> C["Stomach: Pepsin, HCl"]
C --> D["Duodenum: Trypsin, Lipase, Amylase"]
D --> E["Small Intestine: Maltase, Sucrase, Lactase"]
B -.-> B1["Starch → Maltose, pH 6.8"]
C -.-> C1["Protein → Peptides, pH 2"]
D -.-> D1["All macromolecules, pH 8"]
E -.-> E1["Disaccharides → Monosaccharides"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style C fill:#fca5a5,stroke:#000
style D fill:#86efac,stroke:#000Why This Works
The digestive system is designed as an assembly line — each enzyme handles a specific step at a specific pH. The mouth starts starch digestion (neutral pH), the stomach handles protein denaturation and initial breakdown (acidic pH), and the small intestine completes digestion of all three macromolecules (alkaline pH).
The pH dependency is not arbitrary — each enzyme evolved to work where it is needed. Pepsin needs acid because its substrate (large protein) needs to be unfolded first (HCl denatures food proteins), and its active site geometry requires protonated amino acid residues.
Common Mistake
Students often write that “digestion of starch begins in the stomach.” This is wrong. Salivary amylase is inactivated by stomach acid. Starch digestion begins in the mouth and pauses in the stomach. It resumes in the duodenum when pancreatic amylase takes over. NEET tests this fact repeatedly.
Memory shortcut — the pancreas is the “master factory” of digestion. It produces enzymes for ALL three macromolecules: amylase (starch), trypsin (protein), and lipase (fat). If a NEET question asks “which organ produces enzymes for all food groups?” — the answer is always pancreas.